This invention relates generally to voltage regulated power supplies, and more particularly to improved circuit means for current limiting and for reducing the power dissipated in the current sensing resistor in a voltage regulated power supply.
Current limiting power supplies utilizing a simple current sensing resistor have the disadvantage that the power dissipated in the current limiting or sense resistor reduces the efficiency of the power supply and usually requires one or more high wattage resistors which can generate considerable heat. When simple attempts are made to reduce this dissipation by effectively adding the base-emitter drop of the series pass transistor to the drop in the sensing resistor, the temperature coefficient of the base-emitter drop in this power stage causes a change in the current limit setting as the power supply warms up. This can be particularly objectionable when the passing stage is a well known three terminal Darlington transistor circuit, where the voltage variation with temperature due to the combined voltage drop of multiple transistors can cause voltage variations of typically 40 percent or more over the operating temperature range of the power supply. Moreover, in high current applications where several passing transistors are connected in parallel, the problems of current limiting each transistor to the safe value of its own prescribed current in case one or more passing transistors fail in an open or short circuit condition, can produce failures in the remaining transistors.
In the past a breakdown prevention diode has been used to remove excessive voltage from the input to an amplifier to prevent damage to the amplifier when the output voltage drop relative to the reference voltage becomes excessive, and is disclosed in, for example, U.S. Pat. No. 3,536,987 issued Oct. 27, 1970 to P. Muchnick. This circuit is used in a current mode and voltage mode power supply operating in the current mode where the voltage across the sense resistor controls the output current. However, in a voltage regulated power supply the purpose of the sensing resistor is generally for the protection of the power supply during an overload condition rather than for current control, and thus the smaller the resistance used as the sensing resistor, the greater the efficiency of the power supply. Those voltage regulated supplies which attempt to limit the value of the sensing resistor normally use the base-emitter drop of the series pass transistor in conjunction with the sensing resistor which, in turn, introduces a voltage, which is temperature dependent. In effect, the series pass transistor during warm up appears to indicate a change in current across the sensing resistor, thereby changing the selected current setting of the current limit circuit.
Accordingly, it is an object of the present invention to provide a novel circuit for protecting a transistorized, voltage regulated power supply from the aforementioned undesirable current limit variations and to provide improved current limiting and temperature tracking performance.
A further object of the present invention is to provide an improved circuit in which the current limit point can be accurately set to a fixed value when the power supply is either cold or hot, due to the improved stability of the current limit circuit.
Another object of the invention is to provide a power supply circuit of improved efficiency which protects against damage of transistors in a multiple series pass circuit due to a failure of one of the transistors.
A further object of the present invention is to provide an improved current limiting circuit for a voltage regulated power supply that is relatively simple in construction and operation, and yet highly efficient in use.